A selective PET or SPECT ligand for the presynaptic element of serotonin (5-HT) neurons has yet to be developed. Once available, such a ligand should greatly facilitate study of 5-HT function in the living human brain. It should also permit detection of subclinical 5-HT neural damage in human populations considered at risk [e.g., individuals exposed to neurotoxic amphetamine drugs such as 3,4-methylenedioxymethamphetamine (MDMA) and fenfluramine]. During the past funding period, this laboratory has synthesized, radiolabeled and evaluated a series of compounds that held promise as selective presynaptic 5-HT PET/SPECT ligands. Compounds tested include several [11C]/[18F]-labeled paroxetine analogs, [11C]-labeled citalopram, fluoxetine, cis-N-,N-dimethyl-3-(2'4'-dichlorophenyl)-indanamine and [123I]-labeled RTI-55. Of these, only [123I]-RTI-55 was found to display sufficiently high specific in vivo binding to be considered useful for quantitative SPECT imaging. Preliminary in vitro and in vivo studies indicate that [123I]-RTI-55 labels the 5-HT transporter with high affinity and that it can be used to detect clinically silent damage of 5-HT neurons in rodents as well as nonhuman primates. The specific aims of this proposal are to 1) develop a unilateral animal model of 5-HT neurotoxicity that will facilitate development of a presynaptic 5-HT PET/SPECT ligand; 2) confirm and extend preliminary studies showing that [123I]-RTI-55 is a promising SPECT ligand for labeling the 5-HT transporter in vivo; 3) use [123I]-RTI-55 to detect subclinical damage of 5-HT neurons induced by MDMA and fenfluramine in nonhuman primates; 4) label RTI-55 with [11C] for PET studies and compare the results with those obtained by SPECT using [123I]- labeled RTI-55 and 5) continued efforts to develop other [11C]/[18F]- labeled PET ligands that will label the 5-HT transporter more selectively than RTI-55 (which also labels the DA transporter). The long-term goals of this research are to develop novel SPECT/PET ligands which can serve as tools for investigating 5-HT function in the normal living human brain. In addition, this research seeks to develop methods for using such ligands to detect subclinical damage of 5-HT neurons in patient populations.